1. Field of the Invention
The present invention relates to the method of operating a gas engine accommodating to sour gas, in which maintenance free, continuous operation of the gas engine for driving a compressor to pressure feed the gas in a gas field scattered in vast wasteland is made possible by the use of sweet gas decreased of hydrogen sulfide to such extent as causing no damage to the gas engine.
More specifically, the invention relates to the method of operating a gas engine accommodating to sour gas, in which sweet gas is created by separating hydrogen sulfide from a part of the compressed sour gas after removal of water and oil from the sour gas produced in a gas field to be pressure fed to a natural gas treating plant, the sweet gas is supplied to the gas engine to drive the compressor for compressing the sour gas, enabling maintenance free, continuous operation under the severe weather condition in a district where utilities such as electricity and water supply are not available and people seldom approach in winter season, and enabling atmospheric pollution preventing treatment by returning the separated hydrogen sulfide into the sour gas without releasing it to the atmosphere.
2. Description of the Related Art
In natural gas markets in the USA, Canada, and so forth, the natural gas is pressure fed by compressors driven by gas engines. Sweet gas not containing hydrogen sulfide has been used as fuel for gas engines in order to evade damage to the gas engines.
However, in recent years, sweet gas not containing hydrogen sulfide tends to be depleted, and the use of sour gas containing hydrogen sulfide is becoming unavoidable.
In the raw natural gas obtained in a gas field is contained frequently hydrogen sulfide and its concentration is often different according to the district where the gas field exist. This hydrogen sulfide generates corrosive substance when reacted with the water in the gas and induces corrosion in the gas engine to cause failure.
Under the situation where sweet gas not containing hydrogen sulfide is being depleted, the use of sour gas containing hydrogen sulfide is becoming unavoidable, and appearance of a gas engine which can accommodate to sour gas is strongly demanded.
On the other hand, when the sour gas is combusted as fuel, combustion gas containing a large amount of sulfur is exhausted from the gas engine, inducing environmental destruction due to atmospheric pollution. Now the situation is that the provision of an expensive exhaust gas treatment apparatus is inevitable for preventing air pollution even in the district where emission regulations are not enforced on the supposition that there also will be enforced regulations in near future.
Therefore, if it is possible to reduce sour gas to sweet gas containing less hydrogen sulfide on the fuel supply side to the gas engines, that makes possible the continued usage of conventional gas engine accommodating to sweet gas, and as the fuel is removed of said sulfur, SO2 buildup after combustion is eliminated, which contributes to the suppression of environmental destruction.
As a means for excluding hydrogen sulfide from sour gas is used an adsorption type, membrane separator type, or combination of both type, and a variety of means are proposed and applied for patent.
Many of the adsorption type are disposable, and in the case of reusable type by reactivation, a heat source or gas for reactivation is needed, which requires time and cost.
On the other hand, a membrane separator type has an advantage in that it enables maintenance free, continuous operation for a prolonged time period when service condition is properly determined.
Inventions concerning the apparatus for making sweet gas utilizing membrane separator are disclosed in U.S. Pat. No. 4,370,150, and U.S. Pat. No. 6,053,965.
The usage of membrane separator type is thought to be most suitable in point of view of that sour gas field is located at a inconvenient place that does not allow permanent residence of people as mentioned above(without any sources of supplying cooling water and electricity, and a place where people can seldom approach during the winter season).
However, in the case of the membrane separator, there is a problem that the separating function can work with high efficiency for high pressure gas, but can not accommodate to low pressure gas at engine start.
Namely, when the membrane type separator is used, as high pressure gas can not be obtained for a while after starting, sweet gas removed of hydrogen sulfide can not be used during that time.
The invention described in the former patent office journal is shown in FIG. 2. As seen in the drawing, sour gas 78 is compressed by a compressor 80 driven by a gas engine 81 to high pressure sour gas 83a to be pressure fed to an external plant. A part of the high pressure gas 83a, which is shown as 83b in FIG. 2 is branched off to be introduced to membrane separator 82, sweet gas 82a obtained therein is supplied to the gas engine 81 which drives the compressor 80. The hydrogen sulfide gas 82b permeated through the membrane in the membrane separator 82 is returned to the suction part of the compressor 80 to be merged with the sour gas 78.
The invention described in the latter patent office journal is shown in FIG. 3. As seen in the drawing, sour gas 85 is compressed by a compressor 90 driven by a gas engine 91 to high pressure sour gas 86 to be pressure fed to an external plant. A part of the high pressure gas 86 is branched off, cooled in a heat exchanger 94 to be reduced to high pressure, low temperature gas, and introduced to a phase separator 95 where hydrocarbons 95a to be merged with the high pressure sour gas 86 are separated. Sweet gas 92a obtained by passing through the membrane unit 92 is reduced to low pressure low temperature sweet gas by the medium of an expansion valve 93, the sweet gas 94a is heated in the heat exchanger 94 and supplied to the gas engine to drive the compressor 90. The hydrogen sulfide 92b permeated through the membrane in the separator unit 92 is returned to the suction part of the compressor 90 to be merged with the sour gas 85.
These two inventions for obtaining sweet gas for operating the gas engine to drive the compressor aim maintenance free, unattended operation for a prolonged period of time, and the features are as follows according to the disclosures.
1. Membrane Material
a) U.S. Pat. No. 4,370,150:
dimethyl silicone membrane, silicone polycarbonate membrane, polysulfone membrane with polyalkylsiloxane.
b) U.S. Pat. No. 6,053,965:
silicone rubber, polyamide-polyether block copolymer.
2. Temperature Adjustment of the Gas to be Treated
a) U.S. Pat. No. 4,370,150: not described.
c) U.S. Pat. No. 6,053,965:
cooled by the adiabatic expansion of treated gas.
3. Operating Method of the Gas Engine at Starting
a) U.S. Pat. No. 4,370,150: not described.
b) U.S. Pat. No. 6,053,965: not described.
In these inventions for obtaining sweet gas for operating the gas engine to drive the compressor by the use of membrane separator, temperature control which is most necessary in the separation by a membrane is not described, and the problem that sweet gas can not be obtained when the compressor output pressure is low for a while after starting is not considered.
Sour gas is frequently a gas saturated with water. The water and hydrogen sulfide in sour gas and the water contained in the air for the combustion in the gas engine cause not only deposit buildup and corrosion resulting in short life of the gas engine but also make the continuous operation impossible.
In the case of systems operated on gas fields in remote districts scattered in vast wasteland where utilities such as electricity and water supply is not always available as mentioned above, sufficient technical considerations should be given for each constituent apparatus to accommodate to the severe continuous operation under severe weather condition.
The present invention is made in light of the problems mentioned above. The object is to provide an operating method of a gas engine accommodating to sour gas and a system thereof, in which the creation and usage of sweet gas for the combustion in a gas engine are enabled by removing hydrogen, water, liquid mist, and oil from sour gas so that the sour gas containing hydrogen sulfide by high concentration can accommodate to the gas engine, thus enabling the continuous operation of pressure feeding of the sour gas, and the removal treatment for creating the sweet gas is performed in a way the environment pollution is not induced.
The first invention of the present invention is an operating method of a gas engine accommodating to sour gas in the operation of the gas engine for driving a compressor to pressure feed sour gas in a gas field, characterized in that
said gas engine is operated on a sweet gas removed of hydrogen and water by an adsorptive separating means at the starting thereof to drive said compressor for obtaining high pressure sour gas,
after said high pressure sour gas is obtained as mentioned above, a properly compressed gas removed of liquid mist, water, and oil by means of a pre-treatment means is obtained from said high pressure sour gas, and said sweet gas is obtained from said properly compressed gas by means of a membrane separator to be supplied to said gas engine for continuous operation.
The invention enables the usage of sweet gas which does not cause problems in the operation of the gas engine for driving the compressor for pressure feeding sour gas from the time the gas engine is started.
That is, the separation of hydrogen sulfide necessary for producing sweet gas is performed through the membrane separator in which the pressurized high pressure sour gas is dealt with in order to perform the separation with certainty and steadiness in stead of performing through the adsorptive separator which is not capable of completely separating hydrogen sulfide, and the adsorptive separator which can perform the separation even when gas pressure is low is used at the start until the high pressure sour gas is produced by the compressor.
Sweet gas obtained through the separation of hydrogen sulfide gas and water from sour gas by adsorptive separation using disposable adsorbent is supplied to the gas engine to drive the compressor when the compressor is started to operate, the operation on said sweet gas is done until the high pressure sour gas is obtained from the compressor, and after that the operation is switched to be done on the sweet gas obtained from the membrane separator.
When the membrane separator is used, liquid mist, water, and oil is removed from the high pressure sour gas produced through the compression of the low pressure sour gas and the temperature is adjusted through treating in the pre-membrane-treatment means to obtain the compressed gas of proper temperature, and sweet gas is obtained from the compressed gas of proper temperature by removing hydrogen sulfide through the membrane separator to be used continuously as fuel of the gas engine after the starting period.
The invention is also characterized in that the hydrogen sulfide separated through a membrane separator is introduced to the compressor together with the low pressure sour gas from a gas field to produce a high pressure sour gas, said high pressure sour gas is removed of liquid mist, water, and oil and adjusted to a proper temperature for membrane treatment by means of a pre-membrane-treatment means to obtain a compressed gas of proper temperature, a part of the compressed gas of proper temperature is returned to the membrane separator, and the remaining large part of the compressed gas of proper temperature is sent to an external plant as sour gas.
The invention describes the treatment of the hydrogen sulfide separated in order to obtain sweet gas from the sour gas. A large part of hydrogen sulfide in the compressed gas already removed of liquid mist, oil, and water and adjusted to a temperature for membrane separation through the pre-membrane-treatment means is permeated through the membrane of the membrane separator, the permeated hydrogen sulfide is returned to the low pressure sour gas from the gas field. The returned hydrogen sulfide is compressed together with the low pressure sour gas to the high pressure sour gas which is removed of liquid mist, water, and water through the pre-membrane-treatment means to be reduced to the compressed gas of proper temperature, a part of which is introduced to the membrane separator and the remaining large part of which is sent to the external plant as sour gas on pressure feeding.
It is suitable that the pre-membrane-treatment means removes the oil mixed in the gas preferably through a primary (and a secondary as needed) oil separator, the temperature is adjusted by an after-cooler, water is removed through an oil separator to obtain the compressed gas of proper temperature removed of liquid mist, oil, and water from the high pressure sour gas.
The invention describes the configuration of the pre-membrane-treatment means, in which mixed oil is completely removed by a primary (and a secondary as needed) oil separator, then cooled by an after-cooler, and further water is removed by an oil separator to obtain dry, compressed gas of proper temperature.
It is suitable that a filter and a heater are provided upstream from the membrane separator so that a minute amount of mixed water and oil are removed and the inlet gas to the membrane separator is heated by the use of the exhaust heat of the gas engine.
In an a preferable embodiment of the invention, a filter with heater is provided upstream from the membrane separator for completely removing a minute amount of mixed water and oil from the compressed gas of proper temperature and heating the gas completely removed of water and oil to a constant temperature most suitable for membrane separation by utilizing the exhaust heat of the gas engine.
It is also suitable that the compressor uses hydrogen sulfide-insolvable mineral oil/synthetic oil as lubrication oil.
By using hydrogen sulfide-insolvable mineral oil/synthetic oil for the lubrication of the compressor, resistivity to the hydrogen sulfide gas returning from the membrane separator and the hydrogen sulfide gas contained in the sour gas supplied from gas field is gained.
The operating system of a gas engine accommodating to sour gas according to the present invention is suitably constituted to use the operating method of a gas engine accommodating to sour gas. The system is an operating system of a gas engine for driving a compressor to pressure feed sour gas in a gas field, characterized in that it comprises a high pressure sour gas producing part for compressing and feeding sour gas by a compressor driven by a gas engine, a pre-membrane-treatment means for obtaining a compressed gas of proper temperature by removing liquid mist, water, and oil from said high pressure sour gas, a membrane separator for obtaining a sweet gas to be supplied to said gas engine by separating hydrogen sulfide from the gas branched off at the outlet of said compressed gas of proper temperature to the outside, the separated hydrogen sulfide being returned to said compressor, and an adsorptive separating means for supplying a sweet gas created by removing hydrogen sulfide and water from sour gas when starting the operation of the system.
The invention describes the constitution of the operating system of a gas engine accommodating to sour gas, which consists of
a high pressure sour gas producing part for producing the high pressure sour gas to be sent to an external plant by compressing sour gas by a compressor driven by a gas engine,
a pre-membrane-treatment means for performing the treatment of removing liquid mist, water, and oil, and adjusting the temperature to obtain compressed gas of proper temperature to create sweet gas from the high pressure sour gas,
a membrane-separating means for obtaining sweet gas from a part of the compressed gas of proper temperature, and
an adsorptive separator for obtaining sweet gas from the low pressure sour gas in the starting of the system, and
is constituted so that a large part of the compressed gas of proper temperature is sent to an external plant as sour gas on pressure feeding, the hydrogen sulfide separated through the membrane separator is directly returned to the compressor to be merged into the low pressure sour gas supplied from a gas field.
The proper gas temperature for membrane separation is preferable to be, for example, in the neighborhood of 40xc2x0 C. at the entrance to the membrane separator in the case of a hollow-fiber membrane of polyimide.
The membrane-separating means is preferably provided with a filter for removing minute amount of mixed oil and water from the compressed gas adjusted to a proper temperature for membrane separation upstream therefrom and a heater for heating the inlet gas to the membrane separator to a constant temperature in order to obtain high efficiency of membrane separation.
Although the system of the present invention is provided with a pre-membrane-treatment means for removing liquid mist, oil, and water and for adjusting the temperature to a proper temperature necessary for membrane-separation, there is provided in addition a preceding step filter consisting of filters for removing a minute amount of mixed oil and a minute amount of water and a heat exchanger utilizing the exhaust heat of the gas engine for heating the inlet gas to the membrane separator in the coldest season upstream from the membrane separator in order to perform positive pretreatment.
The pre-membrane-treatment means is preferably composed of a primary (and a secondary as needed) oil separator for removing oil from the high pressure sour gas, an after-cooler for controlling the temperature to a proper temperature, and a drain separator for removing water.
A preferable composition of the pre-membrane-treatment means includes a primary (and a secondary as needed) oil separator for separating mixed oil and liquid mist, an after-cooler for adjusting the gas temperature to a proper temperature for membrane separation, and a drain separator for removing water, so that the pre-treatment is carried out to obtain the compressed gas of proper temperature before membrane separation removed of the included oil, solid mixture, and the like to ppm order from the high pressure sour gas compressed by the compressor, the temperature being controlled in the range in which separation efficiency of the membrane in use is high in order to attain good membrane separation.
It is suitable that the compressor is composed so that preferably hydrogen sulfide-insolvable mineral/synthetic oil is used as lubrication oil.
As the hydrogen sulfide separated through the membrane and the sour gas supplied from the gas field containing hydrogen sulfide are sucked and compressed by the compressor, said hydrogen sulfide-insolvable mineral/synthetic oil is used as lubrication oil which is resistant to hydrogen sulfide.
The gas engine is preferable to be provided with a purging circuit including a filter which also serves to neutralize acid for preventing deterioration of the engine oil.
The gas engine employed in the system is operated under severe conditions due to the environment and particularly stable and continuous operation is required. The invention was made to suffice the specific conditions mentioned above, and the engine is provided with a filter circuit as described above for preventing deterioration of engine oil.